1. Field of the Invention
The invention relates to OFDM telecommunications, and in particular, to a speed estimation method detecting relative speeds between a transmitter and a receiver transmitting symbols by OFDM sub-carriers through a channel.
2. Description of the Related Art
According to Orthogonal Frequency Division Multiplex (OFDM) standard, data is delivered separately through a plurality of sub-carriers with guard interval insertion, efficiently overcoming multi-path interference issues. In a mobile telecommunication system, such as wireless cellular network, a moving transmitter (or a receiver) is accommodated in a mobile station and the receiver (or the transmitter) is accommodated in a base station of a telecommunication system. A channel for mobile communication is challenged by time varying multi-path interferences. The channel response has a variation rate referred to as coherence time Tc. The coherence time Tc is a period inversely proportional to Doppler shift frequency fd estimated between the transmitter and the receiver. Conventionally, the formula is provided as:
                              T          c                =                  0.423                      f            d                                              (        1        )            
The higher the Doppler shift frequency, the shorter the coherence time. The Doppler shift frequency is also associated with the relative speed between the transmitter and receiver, proportional to the frequency of transmitted signal. Thus a formula is given as:
                              f          d                =                              f            c                    ·                      v            c                                              (        2        )            
Where c is the speed of light, and fc is the transmitted frequency. With formula (2), the relative speed v can be estimated since fc and c are known, and the fd is detectable.
As described in J. Cai, W. Song, and Z. Li, “Doppler Spread Estimation for Mobile OFDM Systems in Rayleigh Fading Channels,” IEEE Tr. Consumer Electronics, vol. 49, issue 4, Nov. 2001, if a transmitter continuously sends a specific pattern while the channel varies with time, the receiver will obtain a correlation value by auto-correlating the received symbols based on a zero order Bessel function expressed as:φ(Δt)=J0(2πfdΔt)  (3)
Where the φ(Δt) is the correlation value corresponding to a delay time Δt. The delay time is a multiple of symbol time Ts:Δt=mTs  (4)
Thus, the correlation value φ(Δt) in formula (3) is also represented as:φ(m)=J0(2πfdmTs)  (5)
Where Ts represents the duration of one symbol time, and m is a positive integer.
Conventionally, speed estimation utilizes complex algorithm and hardware, which is deemed ineffective. To estimate relative speed while either the transmitter or the receiver is moving in communication, a more efficient method is desirable.